1. Field of the Invention
The present invention relates to a flat panel type image display apparatus adapted to display an image in such a manner that electrons are made to be emitted from electron-emitting devices provided in a rear substrate, and that phosphor layers provided in a front substrate is excited by the electrons to emit light.
2. Description of the Related Art
In recent years, a field emission display (FED), a display apparatus including surface conduction type electron-emitting devices, and the like, have been known as flat display apparatuses having a vacuum envelope of a flat panel structure.
The FED and the display apparatus including surface conduction type electron-emitting devices have a vacuum envelope in which peripheral portions of front and rear substrates arranged opposite to each other at a predetermined interval via spacers are joined by a rectangular frame-like side wall, and the inside of which is evacuated.
Phosphor layers of three colors and a metal back covering the phosphor layer are formed over the inner surface of the front substrate. On the inner surface of the rear substrate, a number of electron-emitting devices as electron emission sources to make the phosphor layer excited and emit light, are arranged in correspondence with each pixel of the phosphor layer. Further, a getter film is formed over the inner surface of the front substrate in order to maintain a high vacuum inside the vacuum envelope.
A voltage higher by several kilovolts than the voltage of the electron-emitting device is applied to the metal back and the getter film, so that an electron beam emitted from the each electron-emitting device is accelerated by the electric field. Thus, the accelerated electron beam passes through the metal back and the getter film, so as to be irradiated to the corresponding phosphor layer. Thereby, the phosphor is excited and emits light so as to display a color image.
In this way, when the high voltage for accelerating the electron beam is applied between the front substrate and the rear substrate which are arranged close to each other, a problem of discharge often arises. When the discharge is caused, a large current flows through the discharge place to result in a problem that the electron-emitting device in the discharge place is damaged.
As a method for solving such a problem, there is known a technique to reduce the discharge damage by such a way that the metal back covering the phosphor layer of the front substrate is electrically divided into small regions, and the resistance between the divided regions is made high so as to suppress the current flowing at the time of the discharge (see Japanese Patent Laid-Open No. H10-326583). Further, a resistance value of the resistor electrically connected between the divided regions is disclosed in Japanese Patent Laid-Open No. 2006-185701.
There is considered a case of providing an electroconductive layer to which a voltage (anode voltage) for accelerating electrons is applied. Here, the electroconductive layer corresponds to one or both of the metal back and the getter. It is preferred that the metal back layer of the image display area is formed over the entire image display area from a viewpoint of making the luminance of a display image uniform. Further, it is preferred that the getter is uniformly formed over the entire image display area from a viewpoint of maintaining the degree of vacuum and a viewpoint of the uniform service life of the electron source.
The electroconductive layer may be arranged in required regions. However, it is difficult to form the electroconductive layer only in the required regions. For example, the electroconductive layer is formed into a desired shape by vapor deposition using a mask. However, since the mask cannot be brought into close contact with the substrate, it is not possible to set the deposition range of the electroconductive layer which is formed outside the image display area. It is possible to make the electroconductive layer surely formed at least in the region where the electroconductive layer must be arranged, by setting the forming region of the electroconductive layer large. The region where the electroconductive layer must be arranged is the image display area. However, it is difficult to make the electroconductive layer surely formed in the image display area, while preventing the electroconductive layer from being formed outside the image display area. Further, in the case where the electroconductive layer is uniformly formed, a large current flows at the time when a discharge is caused between the electroconductive layer and the rear substrate. Therefore, it is preferred that the electroconductive layer is not provided as one large electroconductive layer, but is provided by being divided into a plurality of electroconductive layers. Here, even when the electroconductive layer is formed outside the image display area, it is also preferred that the electroconductive layer outside the image display area is provided by being divided into a plurality of electroconductive layers. However, there is a problem that when the plurality of electroconductive layers are completely electrically isolated, the potential of the electroconductive layers becomes unstable. Therefore, the present inventors have performed investigation for the purpose of adopting a structure in which the plurality of conductive layers outside the image display area are electrically connected by resistors. Specifically, the present inventors have performed investigation about a structure in which mutually adjacent ones of electroconductive layers are made to overlap with respectively different portions of a common resistor, and to be electrically connected to the resistor. As a result of the investigation, it was found that a specific problem arises when the structure is adopted. The problem will be described below. FIGS. 9A and 9B schematically show contact states of the electroconductive layers and the resistor outside the image display area. FIG. 9A shows an example in which the electroconductive layer formed by vapor deposition is widely diffused. FIG. 9B shows an example in which the diffusion region is not expanded so much. As shown in FIGS. 9A and 9B, when a resistor 160 has a simple rectangle shape, a contact area between the resistor 160 and an electroconductive layer 135 is greatly influenced by the diffusion degree of the electroconductive layer formed by vapor deposition. That is, when the electroconductive layer is formed by being widely diffused as shown in FIG. 9A, it is possible to widely secure the contact area between the resistor 160 and the electroconductive layer 135 as shown by the broken lines. On the other hand, when the electroconductive layer is not so widely diffused as shown in FIG. 9B, the contact area between the resistor 160 and the electroconductive layer 135 is reduced. In this way, the vapor-deposited area outside the image display area is unstable. Thus, in the case where the resistor 160 has the simple rectangle shape, the mutual contact area between the resistor 160 and the electroconductive layer 135 for making the current surely flow from the resistor 160 to the electroconductive layer 135 and vice versa, is greatly influenced by the degree of vapor deposition of the electroconductive layer.
Therefore, an object of the present invention is to provide an image display apparatus in which the overlapping area between the electroconductive layer and the resistor outside the image display area is hardly influenced by the size of the forming region of the electroconductive layer.